The present invention relates to ceramic compositions, ceramic powders and ceramic materials for producing capacitors with a high dielectric constant (K), a low dielectric loss tangent (tan xcex4), a low capacitance change with temperature (temperature coefficient of capacitance, TCC), a good resistivity (p) and a small grain size. The ceramic compositions, ceramic powders and ceramic materials of the present invention are useful for the applications in ceramic chip capacitors, barrier layer capacitors including grain boundary barrier layer capacitors and reduction-reoxidation type semiconductive capacitors, multilayer ceramic capacitors (MLCCs), low temperature co-fired capacitors (LTCCs), polymer/ceramic capacitors, and modules integrated with resistors and/or inductors for telecommunications, military and automotive applications, and data processing.
The discovery of BaTiO3 opens the era of the electronic ceramics in 1930s. BaTiO3, a perovskite structure, is now one of the major components most frequently used in the formation of ceramic dielectrics because of its high dielectric constant. Cubic perovskite BaTiO3 is obtained at high temperature; tetragonal BaTiO3 starts to form at a temperature of about 120xc2x0 C.; orthorhombic BaTiO3 is obtained at a temperature of about 0xc2x0 C.; and rhombic BaTiO3 is obtained at a temperature of about xe2x88x9280xc2x0 C. The phase transition point for the cubic-to-tetragonal transition is specially called the Curie point (about 120xc2x0 C.). Below Curie point, BaTiO3 has ferroelectric characteristics. The dielectric constant of BaTiO3 at the Curie point of 120xc2x0 C. can reach a peak value as high as 10,000, but it decreases rapidly as the temperature deviates from Curie point. Chemical additives or so-called xe2x80x9cshiftersxe2x80x9d have been applied to BaTiO3 to move the Curie peak value to improve the capacitance and to smooth the Curie peak so as to obtain a lower TCC close to room temperature.
The perovskite structure of BaTiO3 has two cation sites: tetrahedral and octahedral. The tetrahedral site or so-called A site is larger and is only suitable for large-sized ions such as Ba2+, Ca2+, Sr2+, La3+ etc. The octahedral site or so-called B site is smaller and suitable for small-sized ions such as Ti4+, Z4+.
Much effort has been taken to ceramic materials with high dielectric constants at room temperature. Two material systems have been the focuses. One is BaTiO3 and the other is the lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 or PMN. Some promising examples of BaTiO3 system include the strontium (Sr)-doped and lanthanum (La)-doped BaTiO3. These doped BaTiO3 have the dielectric constants of 10,000 to 19,000, Morrison et al., J. Am. Ceram. Soc., 81 [7] 1957-1960 (1998) at room temperature. PMN is also a perovskite structure and a ferroelectric material. Dielectric constant of PMN has a broad maximum at the Curie temperature of around 0xc2x0 C. With 10% PbTiO3 doping, the Curie point can be moved toward room temperature and the dielectric constant can reach 29,000, Yan et al., J. Mater Res., 4 [4] 930-944 (1989).
Dielectrics with dielectric constant exceeding 1000 are based on ferroelectric materials and are more sensitive to temperature, field strength and frequency than lower-permittivity dielectrics. The Electronics of Industries Association (EIA) of the United States has devised a scheme for specifying the variability of capacitance with temperature in the range of practical interest. Class II capacitors possess high dielectric constants. Y5V, Y5U, and Z4V are examples. These capacitors with high dielectric constants also show a high TCC of greater than xc2x120%.
Barrier-layer capacitors i.e. Class III capacitors are another form of electroceramics with a high dielectric constant. Most materials containing TiO2 become conductive on firing in reducing atmospheres. One of the barrier-layer capacitors, the reduction-reoxidation type, is formed by annealing the reduced TiO2-containing electroceramic in air or oxygen, which results in a surface layer of high resistivity and a central portion of semiconductive material for a dielectric layer. Alternatively, each semiconductive grain may be surrounded by an insulating barrier layer. High dielectric constants of 50,000 to 100,000 were reported for BaTiO3-based materials, G. Goodman, Advances in Ceramics, Amer. Ceram. Soc., 1, 215-231 (1981). For most barrier-layer capacitors, the effective permittivity is 10,000 to 20,000 for SrTiO3 and up to 50,000 for BaTiO3 electroceramics. Although the SrTiO3-based ceramic materials are more stable with respect to temperature than those based on BaTiO3, they have a TCC within xc2x120% over a xe2x88x9220 to +85xc2x0 C. range, Moulson and Herbert, pp. 262 in Section 5.7.4 of Electroceramics: Materials, Properties, Applications, Chapman and Hall, 1990.
Multilayer ceramic capacitors (MLCCs) possessing high capacitance can be engineered into passive components that are readily attachable to the substrates used in the electronic industry. The major cost of MLCCs comes from the palladium (Pd) composition of the electrode. MLCCs with Agxe2x80x94Pd electrodes are sintered in air. By replacing the precious metal electrodes with nickel or its alloy electrodes, i.e. base metal electrodes (BMEs), the cost of MLCCs can be greatly reduced. To protect Ni or Ni alloy from oxidation, the BME approach for MLCCs requires to be processed in a reducing atmosphere, an inert atmosphere, or a controlled oxygen pressure. MLCCs can keep their dielectric properties at low-oxygen densification only if they are processed in the presence of a sufficient concentration of acceptors. BME dielectrics with high dielectric constants usually display high TCC values. A considerable development is needed to explore for the BME dielectrics to resist reduction. [(Ba0.85Ca0.15)O]1.01(Ti0.9Zr0.1)O2 is a representative with its stability against firing under reducing conditions, page 258 in Section 5.7.4 of Electroceramics: Materials, Properties, Applications, Chapman and Hall, 1990.
Ceramic capacitors can be miniaturized by use of a dielectric ceramic material with a high dielectric constant (K) or by decreasing the thickness of dielectric ceramic layers. However, the grain size of multi-doped ceramic dielectrics fabricated by the conventional solid-state reaction is typically more than 5 xcexcm. Thus, if the thickness of dielectric ceramic layers is decreased to 10 xcexcm or below, the number of crystal grains present in each layer is considerably decreased, resulting in poor reliability of ceramic capacitors. The Curie maximum of the multi-doped (Ba0.87Ca0.13)(Ti0.88Zr0.12)O3 dielectric is higher at the larger grain size, D. F. K. Hennings, B. Schreinemacher, and H. Schreinemacher, J. the Europ. Soc. 13, 81-88 (1994). The Curie peak is lowered to less than 4,000 and broadened at the grain size of 3 xcexcm. To enhance the capacitance, the advanced dielectric green sheets for MLCCs need to have a thickness of less than 5 xcexcm. With the thinner multiple-stacked layers of a high-K material, MLCCs with high capacitance can be obtained. As the ceramic sheet of MLCCs is less than 5 xcexcm, the grain size of the high-K material is requested to be even smaller. However, microstructures of the current barrier layer capacitors are large-grained to maximize capacitance because the capacitance generally increases with the increase of grain size. It has been an inherent problem to have high-K capacitors with small grain size.
U.S. Pat. No. 4,987,107, issued on Jan. 22, 1991 to Narumi et al., discloses a ceramic composition for reduction-reoxidation type semiconductive capacitors, which comprises barium titanate or barium titanate and strontium titanate, and has a breakdown voltage of not less than 900 V, an insulation resistance of not less than 1010 xcexa9, an electrostatic capacity per unit surface area of not less than 0.06 xcexcF/cm2, and a low dielectric loss tangent, tan xcex4, of not more than 2.0%. This ceramic composition is not satisfactory on the grain size required for MLCCs having thickness of less than 5 xcexcm in each ceramic sheet. Also, the dielectrics obtained from this ceramic composition have higher TCC values.
Therefore, there is still a need to develop a ceramic composition which permits production of a capacitor possessing a fine grain size particularly required by MLCC materials, and exhibiting a high dielectric constant, a low dielectric loss tangent, a low TCC, and a high resistivity. The present invention provides ceramic compositions fitting the aforementioned needs by incorporating lead into multi-doped BaTiO3 ceramics.
Accordingly, one object of the present invention is to provide a ceramic composition that has the following general formula (I):
(1xe2x88x92w)[(BaaSrbPbm)Lax](TiyMgz)O3+wNb2O5xe2x80x83xe2x80x83(I) 
wherein a is 0.5 to 0.98, b is 0 to 0.3, m is more than zero to 0.3, x is 0 to 0.05, y is 0.90 to 0.995, z is 0.005 to 0.1, and w is 0 to 0.05 in weight ratio. The ceramic composition can provide a ceramic material with a dielectric constant of about 20,000 to about 55,000, a low dielectric loss tangent (tan xcex4) of about 0.05 to about 0.25, a TCC value of about xe2x88x9215% to about 10% at a temperature of xe2x88x9255xc2x0 C. to 150xc2x0 C., a resistivity of about 106 xcexa9xc2x7cm to about 109 xcexa9xc2x7cm, and with a grain size of about 0.5 to about 3.5 xcexcm, useful in the production of capacitors and modules. The ceramic material of the present invention with excellent dielectric properties can be obtained at low cost with cheap raw materials.
The second object of the present invention is to provide a ceramic powder with the chemical composition of formula (I), preparing from carbonates, oxides and/or hydroxides of barium (Ba), titanium (Ti), magnesium (Mg) and optionally strontium (Sr), lanthanum (La) and niobium (Nb), and lead titanate (PbTiO3) and/or lead oxide (PbO), wherein the atomic ratio of lead, based on the total amount of Ba, Ti, La, Mg, Sr, Pb and Nb is more than zero to 0.158.
Still, a further object of the present invention is to form a glass/BaTiO3 composite with aids of the ceramic powder of formula (I) and a glass component selected from the group consisting of a low-firing-temperature glass, lead glass or a combination thereof via reduction-reoxidation procedures, useful for producing low temperature co-fired capacitors with dielectric constants of about 360 to about 2,500.